Research progress on material development and forming technology of high mechanical similarity 3D printing coal-like rock

Accurate, efficient, and repeatable fabrication of specimens that match the mechanical properties and structural characteristics of real rock masses is critical for overcoming the bottlenecks of conventional rock mechanics testing. Specimens fabricated via sand-powder 3D printing exhibit mechanical properties similar to those of soft rocks such as coal and sandstone, making this technique an important approach for the physical reconstruction of coal masses. To address the low strength and limited application scope of 3D-printed coal-like specimens, comprehensive investigations of the entire printing process was conducted. The influences of matrix materials, forming parameters, and post-processing procedures on the macroscopic mechanical properties and microstructural characteristics of the specimens are systematically analyzed. Two enhancement and modification methods were proposed based on close packing theory and composite material reinforcement theory, and high mechanical-similarity composite coal-like materials for 3D printing were developed. The findings suggest that, with regard to matrix materials, the utilization of manufactured sand with rough surfaces and complex morphologies can enhance the mechanical interlocking between particles. Among forming parameters, the appropriate layer thickness, binder saturation, and curing agent concentration can improve the interfacial bonding strength between particles. In post-processing, extending the curing time and adopting drying methods with suitable temperatures both contribute to increasing specimen density and strength. In the realm of enhancement modification, the optimization of particle size distribution facilitates the establishment of a robust skeletal structure, reinforced by coarse particles and filled by fine particles, thereby significantly improving density and mechanical properties, while the incorporation of fiber reinforcement materials can inhibit crack propagation through cracking resistance and bridging effects, enhance stress distribution, and increase peak strength. The findings reveal the structure-property relationships between the macroscopic mechanical properties and the material composition and microstructure of 3D-printed coal-like specimens. A high mechanical-similarity composite 3D-printed coal-like material, together with its component compositions and mix ratios, was developed for simulating coal-measure strata. The mechanism of the particle gradation and fiber reinforcement for coal-like materials were elucidated. These results provide essential methods and techniques for the accurate, efficient, and batch fabrication of fractured coal-rock mass specimens (models) employed in coal–rock mechanics and physical model testing.